A .375 Ultra is my preference, but any difference in terms of practical terminal performance between the two is mostly imaginary, particularly if both are fired from rifles with 20" barrels. The question then is, what is your experience with powerful rifles? No offence intended, but usually those asking for advice concerning this cartridge or that, don't have much shooting experience, and a full powered, large case .375 is a tough teacher. The uninitiated find they kick and bark, and they are intimidatingly expensive to shoot enough to gain competence with. Are you a handloader? If so you can ease your way up the power scale, and along the way discover loads that are practical for things other than buffalo. If not, I suggest you consider starting with a milder cartridge with a good reputation for versatility. Certainly the .30/06 comes to mind, as does the .280 Remington, or the .270 Winchester.